The present invention generally relates to optical detection apparatus, and more particularly to a waveguide type optical detection apparatus which is applied to several optical integrated circuit units such as an optical disk pickup, an optical spectrum analyzer photodetector array or the like.
Generally, an optical waveguide, or a waveguide in which a light-transmitting material is used for transmitting information in the form of electric signals from point to point through the material, is commonly used for detecting a focus position in an optical disk pickup, for positioning a movable object, and for other purposes. A conventional waveguide optical detection apparatus, as disclosed, for example, in Japanese Published Patent Application No. 63-71946, provides an example of an optical waveguide apparatus which is built for detecting a focus position. On such an optical waveguide, a grating coupler is provided for transmitting in the air a light beam from a light source onto an optical disk and introducing a light reflected on the surface of the optical disk back to the waveguide. Also provided on the optical waveguide are a set of two adjacent photodetectors which are located on one end surface of the waveguide to receive a light beam sent from the grating coupler. In the case of such a conventional apparatus, the accuracy of the detection greatly depends on the width of the area between the adjacent photodetectors, and on the distance from the photodetectors to the grating coupler. To achieve better accuracy for the optical detection apparatus, it is necessary to either make the photodetector-to-coupler distance longer, or make the width of the area between the adjacent photodetectors narrower. The extent of the above described distance, however, is limited due to the design and overall size of the optical detection apparatus, and therefore it is not possible to have a distance which exceeds a prescribed maximum distance. The above described width is also limited due to the physical restriction inherent in developing a closely-aligned photodetector design on the optical waveguide. Therefore, there still remains the need to develop a new improved waveguide type optical detection apparatus which offers better detection accuracy.
In addition, a conventional waveguide optical apparatus which is adapted to an optical disk pickup unit is disclosed, for example, in an engineering research report entitled "AN INTEGRATED-OPTIC DISK PICKUP DEVICE", OQE85-72, pp. 39-46, Shingaku Giho, Vol. 85, No. 136, issued on Sept. 17, 1985 by the Institute of Electronics and Communication Engineers of Japan (IECEJ). Such a waveguide optical apparatus provides a tiny, ligthtweight optical disk pickup unit which can be produced for experimental use. However, in the conventional waveguide optical detection apparatus disclosed by this published article, there also still remains some problems which must first be resolved before such a pickup unit can be manufactured for practical use. Hence, in recent years, it has been desired that further improvements be developed for an optical disk pickup unit and relevant units which can be manufactured for practical use.
First, referring to FIG. 1, a description will be given of the above-discussed conventional waveguide optical apparatus which is applied to an optical integrated circuit for an optical disk pickup head. In this conventional apparatus, a silicon substrate 31, a buffer layer 32 and an optical waveguide layer 33 are laminated together into a thin-film integrated circuit structure. A semiconductor laser diode 34 of a light source is provided on an end surface of the silicon layer 33 for generating a laser beam, and the beam from the light source 34 is transmitted through the waveguide 33. The light beam traveling through the waveguide 33 passes through a grating 35 and a grating coupler 36, and is diffracted so that a converging light is propagated in the air until it reaches a surface of an optical disk 37. The light is then reflected on the surface of the optical disk 37 back to the grating coupler 36 and the light is again introduced into the waveguide. Then the light is diffracted by the grating 35 so as to split into two separate light beams, which are received by a set of two adjacent first photodetectors 38a, 38b, and a set of two adjacent second photodetectors 38c, 38d, respectively.
Electric signals outputted from these photodetectors when exposed to light are picked up through a logic circuit connected to suitable electrodes of the photodetectors, as shown in FIG. 1. These outputs include a readout signal S, a focusing error signal Fo and a tracking error signal Tr. The signals thus obtained from the photodetectors are expressed as follows: EQU Fo=(S.sub.38a +S.sub.38d)-(S.sub.38b +S.sub.38c) EQU Tr=(S.sub.38c +S.sub.38d)-(S.sub.38a +S.sub.38b) EQU S=S.sub.38a +S.sub.38b +S.sub.38c +S.sub.38d
In these formulas, S.sub.38a, S.sub.38b, S.sub.38c and S.sub.38d are output signals of the photodetectors 38a, 38b, 38c and 38d, respectively. Concerning the focusing error signal Fo, when the optical disk 37 moves away from the pickup unit, the value of the Fo becomes negative, or the Fo is smaller than 0, and on the contrary, when the optical disk 37 approaches the focus position of the pickup unit, the value of the Fo becomes positive, or the Fo is greater than 0. Therefore, this allows for appropriate detection of the focus position by using the conventional apparatus.
However, the conventional waveguide optical apparatus as shown in FIG. 1 has light-insensitive areas between the adjacent photodetectors 38a, 38b and between the adjacent photodetectors 38c, 38d. In the light-insensitive areas, light beams being sent from the grating 35 cannot be received by the photodetectors because the light-insensitive areas are located where no photodetection function is provided. Usually, the width of the light-insensitive area between the adjacent photodetectors ranges from approximately 5 to 10 microns, and it is practically impossible to make this light-insensitive area width negligible or zero.
For example, when detection of the focusing error signal is carried out with the light from the light source 34 being focused exactly on the optical disk 37, the light beams are thrown on the light-insensitive area between the photodetectors 38a and 38b as well as on the light-insensitive area between the photodetectors 38c and 38d. If the beam diameter is smaller than the light-insensitive area width, with the optical disk deviating from the focus position, the value of the Fo becomes zero and the focusing error signal is insensitive. And, if the beam diameter becomes almost at the same level as the light-insensitive area width, such "light-insensitive" condition of the focusing error signal is eliminated. However, when the optical disk deviates from the focusing position, the amount of change in the focusing error signal becomes excessively small, which worsens the sensitivity of the optical detection apparatus.
Since the light-insensitive areas cannot be made narrower than the minimum level, an immediate solution to the problem is to enlarge the distance from the grating 35 to the photodetectors 38. However, such a solution requires the development of a larger optical disk pickup design, which is inconsistent with the desired development of a tiny, lightweight integrated optic unit.